The EU sets the classification of medical devices is based on MDD93/42/EEC (not GMDN) - just see and follow the rules in Article 9 in the link:
GMDN gives a unique number to a device based on the description and its associated definition e.g. "Medical Air Terminal Unit" is defined as "A device that is a component of a medical gas pipeline system........................................", i.e. not a classification. You have to pay for the service to get an actual number and the rest(i.e. become a member). However if you are canny and use the Australian TGA site:https://www.ebs.tga.gov.au/ebs/ANZTPAR/PublicWeb.nsf/cuDevices?OpenView
Details of many devices are there including GMDN and supplier classifications (be careful old registrations might give out of date GMDNs - since they get regularly updated or superseded). As to ISO 15225 - it's a copyright publication £136 from BSI (half price if a member).
I hope this helps.